An if statement is a conditional statement that determines if the condition is true, and if it is true, executes the code inside the block. Syntax: if (condition) {// Code to execute when condition is true}. Where condition is an expression and the code block is surrounded by curly braces {}. The if statement can be used nested, the else statement is used to specify the code to execute when the condition is false, and the else if statement allows to check multiple conditions and execute a block of code that corresponds to the first condition that is true.

In JavaScript for... in and for The main difference of the loop is: Content: for... in traverses an object property or index, while for... of traversing sets of elements or iterable elements. In order: for... in an indefinite order, but for... The order of is stable. Variable type: for... The in variable holds the property name, while the for... The of variable holds the element value. Termination: for... After traversing its own property terminates, and for... Terminates after traversing all elements.

In JavaScript, const is used to declare constant variables whose values cannot be reassigned while the program is running. Uses include: 1) Prevent variable reassignment; 2) Identify constants that do not change. Its syntax is const variableName = value.

The includes() method in JavaScript is used to check whether an array or string contains a specific element or substring. Depending on the specified start parameter, it starts looking at a specific position in the string or array. Returns true if found; If not found, false is returned.

There are two ways to implement a countdown timer in JavaScript: setInterval() : Creates a timer that repeatedly calls the function every specified millisecond. setTimeout() : Calls the function only once, delaying it for a specified time.

The Math.random() function generates random floating point numbers between 0 and 1. To generate random numbers within a specific range, you can use the following steps: 1. Set the minimum and maximum values. 2. Multiply the range width and add the minimum value. 3. Use Math.floor() to round down to get an integer.

slice is a JavaScript method used to extract a range of elements from an array or string. Syntax: array.slice(start, end), where start is the start index (included) and end is the end index (not included). Usage methods include: extracting subarrays/substrings, copying arrays/strings, extracting elements from the beginning/end, and using step parameters. The operation does not modify the original array and creates a new copy.

The differences between slice and splice in JavaScript are as follows: slice() returns a new copy of the array without changing the original array; splice() modifies the original array. The syntax of slice() is slice(start, end) and the syntax of splice() is splice(start, deleteCount,... items). slice() copies elements from the specified location, and splice() removes or replaces elements from the specified location.

JavaScript's Splice method allows you to delete or insert elements from an array, returning a new array containing the deleted elements. The command can be used as follows: Delete an element: arr.splice(start, deleteCount) Insert an element: arr.splice(start, 0,... items) replace the element: arr.splice(start, 1,... items) Deletes elements from the end: arr.splice(-1, deleteCount) Deletes elements from the beginning: arr.splice(0, deleteCount)

Slice is a character string substring extraction method with the syntax str.slice(start, end), where: start is the start index (included), end is the end index (not included), and the return is a substring within the specified range. If start is not specified, the substring starts from the beginning, and if end is not specified, the negative index is the reciprocal index, and the index outside the range is truncated to 0 or the string length.

Slice is used to extract a copy of an array without modifying the original array, and it returns elements within the specified index range. Splice is used to modify arrays. It removes elements from the specified index, inserts new elements, and returns the array of removed elements.

In JavaScript, you can generate random integers between m and n using the following steps: Use Math.random() to generate a random number between 0 and 1. Multiply the result by the range (n-m + 1). Round the result to an integer. Add m to the result.

import and dependjs are both syntax for loading external modules in JavaScript. import is supported in all modern browsers, follows the ECMAScript module specification, loads modules statically, imports into the current scope, and generally performs better than dependjs. dependjs is supported only in Node.js, adheres to the CommonJS module specification, loads modules dynamically, imports into global scope, and is more suitable for situations where a large number of modules need to be loaded at run time.

The replace() method in JavaScript is used to find and replace a specified character or substring in a string, using string.replace(replaceWhat, replaceWith[, count]). It can perform string substitution, regular expression substitution, partial substitution, find and replace functions, and global substitution.

NaN means "non-numeric" in JavaScript, and is generated when a string is converted to a number in the following cases: non-numeric, no-value arithmetic, and NaN arithmetic. NaN cannot be compared conventionally; the Number.isNaN() function is used. NaN is often used to handle data that cannot be represented as a number, allowing a program to give a reasonable response if it encounters invalid data.